These studies have involved the delineation of the distribution and diversity of mouse mammary tumor viral (MMTV) and MMTV-related genetic information in rodents and its correlation with the etiology of mammary carcinoma. Major accomplishments during the past year include: (1) the isolation, from the RNA genomes of highly oncogenic MMTVs, of a subset of sequences that are detected (both by liquid hybridization and restriction endonuclease techniques), only in the DNA of mammary tumors that arise early in life of several mouse strains, and the characterization of these tumor associated sequences (TAS); (b) the use of liquid hybridization and restriction endonuclease techniques as an adjunct in the detection of metastatic mammary tumor cells; (c) the delineation of the diversity of mammary tumor viral genes that exists within the genus Mus and the species M. musculus; (d) a demonstration of the diversity of endogenous MMTV genes among colonies of the so-called "inbred" C3H mouse strain and the presentation of evidence for the recent (within the last few decades) de-novo integration of MMTV sequences as endogenous provirus; (e) the first demonstration that the rat genome contains MMTV-related information.